Method of refining light oil products



Oct. 18, 1949. w. o. KEELING METHOD OF REFINING LIGHT OIL PRODUCTS 2 Sheets-Sheet l Filed April 9, 1945 DmHm HNDR a N I u R E mK E v O m m L M 3 Nv. 0% miam o/ S mm. mv M235@ Nm Wm. dmmzmozoo Oct. 18, 1949. w. o. KEELJNG METHOD OF REFINING LIGHT OIL PRODUCTS 2 sheets-sheet 2 Filed April 9; 1945 O. Kee/.ING

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MOWWNNA BY M s ATTORNEY Patented Oct. 18, 1949 METHOD OF REFINING LIGHT OIL l PRODUCTS William 0. Keeling, Pittsburgh, Pa., assignor to Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware Application April 9,1945, Serial N0. 587,255

8 Claims. l

The present invention relates to a process for recovering benzene of a high degree of purity from admixtures'with other miscible hydrocarbons, and especially pertains to an improved method for separating benzene from therewith miscible hydrocarbons having boiling temperatures close to that of pure benzene.

The bulk of the benzene now produced in this country has its source in coke-oven light oil. Benzene so produced may be associated with other compounds, such, for example, as those familiarly known as the aromatics, unsaturated, paraillns, naphthenes, and sulphur compounds. The separation of benzene from these compounds is normally carried out by chemical treatment and distillation. Often such methods fail to yield a benzene sufficiently free of these associated compounds to make it desirable for certain uses, such, for example as nitration or chlorination; The present invention is particularly applicable for the separation of benzene of a high degree of purity from mixtures containing varying proportions of the above-mentioned materials and especially from such parafnic and cyclic hydrocarbons as n-pentane, cyclopentane and the corresponding hexanes, heptanes, and the like, having physical characteristics which make them difficult of separation with the equipment generally used p in the art. These compounds may be present to the extent of about two to three per cent, their concentration depending on various factors. some of which are the type of coal carbonized, coking time and absorbent oil employed. Not only is the present improvement of great utility in vthe refinement of benzene obtained from coke-oven light oil, but it is also of advantage in the refinement of benzene derived from petroleum, water gas tar, drip oils or from special synthesis.

A primary object of the present invention is to provide a simple and effective method whereby benzene of a high degree of purity can be continuously separated from other hydrocarbons with which it is admixed in consequence of its method of formation or of its recovery.

Another object of the invention is to provide a method by which benzene of high degree of purity may be separated from impurities by a combination of different solvents to obtain a high recovery at a comparatively low cost.

A further object of the invention is to provide a method by which benzene of a high degree of purity may be separated from impurities by a combination of crystallization and azeotropic distillation.

A further object of the present invention is to 2 provide a simple and effective method whereby benzene of a high degree of purity can be continuously separated from admixture thereof with associated hydrocarbons or other compounds having boiling points that are either higher or lowerthan that of pure benzene and particularly from paraflinic orl other compounds having boiling temperatures close to that of pure benzene or which may form azeotropic mixtures therewith.

A further object of the present invention is to provide a simple and effective method whereby benzene of a high degree of purity can be continuously separated from admixtures with associated hydrocarbons by crystallization.

A further object of the present invention is to provide a simple and effective method whereby liquids miscible with benzene are usable as diluents and as carrying agents for the benzene crystals formed when impure benzene is chilled and also as direct or indirect chilling agents in a purification process employing a -crystallization step.

The invention has for further objects such other improvements and such other operative advantages or results as may be found to obtain in the method hereinafter described or claimed.

According to the present invention an impure benzene product is subjected to a crystallization step after there has been admixed therewith a preferred quantity of an extraneously derived and therewith miscible liquid which Will not crystallize from the admixture Within the temperature range employed in the step for crystallizing the benzene. The admixture is introduced into a chilling zone through which it is passed at a velocity adequate to crystallize benzene therefrom and to producea flowable slurry, which comprises preferably a dispersion of relatively small, individual benzene crystals uniformly distributed through-` out `the carrier liquid, and which continuously passes therethrough, said Velocity always being suflicient to retain the crystallized benzene in suspension in a mother liquor, thus keeping the Walls of the chilling apparatus substantially free of benzene incrustations. Upon leaving the chilling zone the slurry is continuously passed through any preferred apparatus for separating the benzene crystals from residual mother liquor, such as a continuous filter or centrifuge. After the separation, the former are washed with either a fresh portion of the miscible liquid or, if preferred, by a small amount of pure benzene previously p roduced by the process, to remove therefrom residual mother liquor. This washing step is carried out without substantial melting of the crystalline benzene and thereafter the purified crystals are crystallization, and then make the further recovery of the benzene by azeotropic distillation.'

Butane or propane or a combination. ot these hydrocarbon solvents may be used to recover themajor portion of the benzene. by chillingv and crystallization, and a minorl portion` or the ben-V zene may thereafter be recovered with an aliphatic alcohol solvent, such as isopropyl alcohol, A

that forms an azeotrope with the benzene and permits pure benzene to be recovered` by azeotropic distillation. Such a combination of s01- vents permits the benzene to crystallize at a moderate freezing temperature and the usev of a comparatively small amount of paraffin hydrocarbon as a solvent, and thereafter the remaining benzene may be recovered with. a comparatively small amount of alcohol solvent toy form a small amount of azeotropic distill-ation liquids.

The invention further provides for the treatment of the separated mother liquor for the recovery of miscible liquid therefrom and also for its `optional recovery from wash liquor residual inthe melted benzene, the both of which can be returned thereafter to the method.

Liquid butane, liquid propane, the petroleum derivative commercially known as Stoddard solvent, and certain of the alcohols can be used as the extraneously derived,A miscible liquid for dilutingv the to-be-purified benzene product before it is passed to the chillingzone. Thesematerials all have solidication points below that of benzene so that they donnot crystallize therewith, and such of these materials as are entrained in the crystalline benzene can be easilyI removed in a. Washing step or by fractional distillation, the lowV boiling materials appearing in the first fraction whereas the Stoddard solvent is recovered asa distillation residue. The-alcohols which form azeotropes with benzene can berecovered. by simple. water extraction of the melted benzene. The low boiling paraffin, hydrocarbons. are particular-ly good solvents for the benzene as well as for the impuritiessuch asv aromatics, unsaturated hydrocarbons, parafiins, naphthenes, cyclopentane, normal pentane and hexane which are often` present as impurities in the benzene.

In the accompanying drawings forming a part of this specification and showing for purposes of exemplication a preferred apparatus and method in which the inventionmay be embodied and practised but without limiting the claimed invention specifically to such illustrative instance or instances:

Figure 1 shows in diagrammatic manner apparatus wherein are practised principles of the presentv invention more specifically when they are practised in combination with benzenel diluents having boiling points substantially different from that of the to-be-purified benzene; and

Figure 2 shows in diagrammatic manner apparatus wherein are practisedl principles ofthe present improvement more especially when they are practised in combination with benzene diluents having boiling points lower than that of the tobe-puried benzene and further employing said diluents as freezing agents in the process by the 4 controlled evaporation of a portion of said diluent from the admixture.

The same characters of reference designate the same parts in each of the views of the drawings.

Referring now to the drawings: Figure 1 particularly illustrates in diagrammatic manner apparatus vsuitable for carrying out the present process in those 'of its applications wherein there are employed miscible liquids, such as butane or the like, Stoddard solvent, or isopropyl alcohol or the like, ofrwhich the boiling points and crystallization temperatures are substantially dilerent from those of benzene.. The apparatus is adapted to operate at pressures above or below atmospheric, where the lphysical properties of the preferred diluent so require.

TheA benzene mixture to be purified enters one section of the proportioning pump 3 through line 2li, from a suitable storage tank (not shown), and is pumped through line 2l to chiller t. The liqueiied butane employed for dilutionl of the benzene mixture is stored in tank I, equipped with level control 2` that governs proportioning pump 3 as a safety device in those cases where the level in storage tank I may become too low. The liquefied butanel is withdrawn from storage tank I through line 26 by proportioning pump 3 whichl sends itv through line 21 to heat exchanger 4 wherein it is given a preliminary chilling by colder liquids from a further step in the process; the cooledv butane leaves heat exchanger il through line 28 andv thereafter enters Chiller 5 wherein said butane is chilled to the preferred temperature by indirect `Contact with further quantities of butane expanded in theV refrigerating jacket of Chiller 5. The expanded butane vapors leave the refrigerating jacket of Chiller 5 through valved line 62, and are returned to compressor I8 through line 63, valveV 6d and line t5. The compressed butane returns to storage tank 5i' through line 'I5 as a liquid after passage through condenser I. Further quantities of butane for use in the refrigerating jacket of chiller 5 are obtained from storage tank 5l through line 53, valve 59 and line 60.

The now chilled and liqueiied butane to be used for dilution and crystallization of the benzenecontaining liquid leaves chilleil 5 through valved line 33 and enters chiller at a point adjacent to the point where the to-be-puried benzene enters the same through line 2l. quantity of the chilled and liquefied butane is intimately and rapidly mixed with said benzenecontaining mixture to convert the admixture from a liquid to a slurry wherein the benzene is in thev form of very fine crystals which are suspended in a mother liquor` comprising the benzene impurities and the liqueiied butane. Scraper conveyor I may be employed to free the walls of chiller '6, whenever necessary, of benzene incrustations. l

The amount of butane introduced into the chiller Sil or into the benzene line 2l will depend upon the method used in purifying the benzene. If the benzene is to berecovered solely by crystallization, then the butane may be equal up to 50 percent by volume of the benzene because a comparatively low temperature minus 20 degrees Fahrenheit may be required (depending on the purity of the benzene) to recover 75 percent of pure benzene in one treatment. If a combination of lcrystallization and azeotropic distillation is to be used, then the amount of butane should notl be more than 2O percent by volume of the benzene and preferably should only be sufficient A sufficient to provide a free flowing slurry of the benzene in the mother liquor crystals. About 15 percent by volume of butane in the benzene will provide a good slurry of benzene crystals when the temperature of the mixture is about zero degrees Fahrenheit and will permit a recovery of pure' benzene of about 50 to 60 percent in one treatment. Since the butane has to be chilled and thereafter has to be separated from the benzene by fractional distillation, it is desirable to use the minimum amount of butane for economy.

The slurry-like mixture is discharged as a flowable mass from chiller 6 through line 22 to the continuous separator 8 wherein the crystallized benzene is continuously separated from the liquid fraction, of which the residues that may adhere to the benzene crystals after said separation'are preferably washed out by means of additional quantities of chilled fresh butane, without substantial melting oi' the benzene crystals. This chilled butane wash liquid is obtained from chiller through valved line 3|. The benzene crystals of high degree of purity obtained after the continuous separation of the original mother liquor and the further washing with additional quantities of chilled butane are discharged from separator 8 through line 23 to melter 9, equipped with stirrers I0. In said melter crystallized benzene is moved by stirrers I0 to bring it quickly to the adjacent surface of a Water jacket to be converted to a liquid by indirect heat exchange with water. The water used to melt the benzene crystals enters the jacket of melter 9 through line 13 and is discharged through line 14 to condenser I9 where advantage is taken of its low temperature to again liquefy the butane vapors after they leave compressor I8. The jacket of the melter can be augmented with coils arranged within the melting chamber itself, if preferred.

The now melted benzene in melter 9 flows through line 24 to pump II, said pump sending the benzene through line 25 to fractionating column I2. By means of indirect heat supplied by steam coil I3 the residual quantity of butane is separated from the highly puried benzene. The butane leaves as a vapor at the top of fractionating column I2 through line 35, is again liquefied in condenser 36 and leaves through line 31 to butane receiver 38. Receiver 38 is equipped with level control 39 that governs pump 48 as a safety device in those cases where the level in receiver 38 may become too low. Pump 40 draws the liquid butane from receiver 38, discharging a regulated quantity for reflux through valves line 42 to fr'actionating column I2. The amount in excess of the reiiux is returned to storage tank I valve 43 and line 44 where it again becomes available for the further separation of benzene from benzene-containing mixtures in accordance with the present process. The benzene, after separation of the residual butane, leaves at the bottom of fractionating column I2 through line 45, cooler 46 and is thereafter sent to a suitable storage tank (not shown) through valved line 48. Valved line 4I and also valved pump line 41 are closed oi during this method of operating the apparatus.

The mother liquor separated originally from the benzene crystals in continuous separator 8 plus the additional quantities of liquefied butane used for washing the benzene crystals free of residual traces of impurities are admixed and withdrawn from said separator 8 through line 32 by pump I4 `which sends said admixture through line 33 to heat exchanger 4 where advantage is taken of the low temperature oi said admixture to cool further quantities of the liqueed butane flowing to chiller 5. From heat exchanger 4 said admixture is discharged through line 34 to iractionating column I5 wherein, by .means of indirect heat supplied by steam coil I6, a separation is effected whereby the butane, in a highly purified state, is recovered from the benzene impurities. The butane leaves as a vapor at the top of column I5 through line 49 and is again liqueed in condenser I1 and thereafter returned to storage tank I through valved line 5I. The necessary reux to column I5 is supplied by proportioning pump 3 through valved line 52. From the bottom of column I5 are withdrawn the impurities, which may contain some benzene, through line 53 and after passage through cooler 54 are discharged to a suitable storage tank (not shown) through valved line 55. Valved lines 55' and 56 are closed ofi' during this method of operating the apparatus.

When albout 50 percent of the benzene is to be recovered by crystallization as explained above, another 35 to 45 percent of the benzene may be recovered by azeotropic distillation. To accomplish this the benzene in the residue or mother liquor from which the butane has been separated will flow through the line 55' to a fractionating still 16. To the impure benzene passing through the line 55 is added isopropyl alcohol through a valved line 11. The isopropyl alcohol forms an azeotropic mixture with the benzene composed of approximately two-thirds benzene and onethird alcohol. Therefore in accordance with the amount of benzene in the residue about one-third of the amount of benzene and by volume of alcohol will be added to the impure product. The benzene-alcohol azeotrope will be distilled overhead through a line 18, condensed in condenser 19 and collected in receiver 80. As the azeotropic mixture passes from the receiver 80 to a separator 8| through a line 82 a suicient amount of water is added from line 83 to the azeotropic mixture to dilute the alcohol and separate the diluted alcohol from the benzene. The benzene may be removed from the separator through a line 83 and the dilute alcohol solution will leave the bottom of the separator through a line 84. The mother liquor containing some benzene will move from the bottom of the still 16 through a line 85. By azeotropic distillation it is possible to recover a nitration grade benzene to the extent of 80 to 90 percent of the benzene in the impure mother liquor. Accordingly the amount of alcohol that is added should not be much more than is required to form an azeotrope with percent of the benzene in the mother liquor coming from the bottom of the still I5.

The combination of crystallization and azeol tropic distillation permits a high recovery of nitration grade benzene with the minimum amount of solvent and the minimum amount of distillation to separate the solvent from the benzene. Furthermore, the amount of refrigeration which is required for crystallizing the benzene and the amount of power required for recovering the parailin hydrocarbons is greatly reduced. The combined solvent process may be carried on continuously and the cooling and fractional distillation operations are comparatively simple so that the apparatus will have a large capacity.

The benzene impurities, which are readily soluble in the butane, after separation from the butane as indicated above may be sent directly to Amotor fuel storage in those instances where the original benzene product being processed .is relatively pure.

In a .modification of the above-described method for Yemploying butane, vor the like, as a chilling agent, the same is chilled in .heat exchanger 4 wherein it is cooled an amount insuiicient to produce substantial crystallization of the benzene vwhen the two are admixed before entering :Chiller 6, the actual crystallization lbeing effected -by indirect contact with further quantities of butane expanded within the jacket' of said chi-ller. The lto-be-treated benzene -owing through line 2l is admixed, previous to its admission to 'Chiller 6, with liqueed butane partially chilled in heat exchanger 4 and flowed through valved line 29 appropriately adjusted 'to permit the preferred quantity to flow to and admix with the benzene of line 2l. In this modiiication valved line 13D leading directly to chiller 6 from chiller 5 is closed. The benzene-butane -admixture is converted from a liquid to a slurry of benzene crystals suspended in mother liquor by indirect contact with expanding butane in the jacket of chiller 5 while flowing through said Chiller. Thereafter, the subsequent operations for the production of a highly purified benzene are 'thessam-e as previously described.

lnthis latter modication of the improvement certain additional lines are required to supply lontane '.to the jacket of chiller l. The necessary butaneis drawn from storage tank v51 through line 53., valve :59, line lili yand enters said jacket of the Chiller through valved line '61. Ihe butane vapors from said jacket are returned .to ycompressor |28 through valved line 63, valve 64, and line 65. rIhe circuit is completed through line and condenser IS :to storage tank 51.

Ammonia, or other refrigerants, can be em.-

ployed for indirect chilling in the jackets of chillers 5 and 6. If preferred, an aqueous sal-ine solution `can .be `.chilled and circulated through the jackets of chillers 5 and t. In this modification of rthe'ref-rigeration system the expansion of butane, ammonia or other preferred refrigerant takes place in a `separate chamber 65, equipped with coils r6? for said saline solution. The preferred refrigerant is supplied from tank 51 through line 53 `and valved line 69 and returned tor compression through valved line 10 and line 455. In'this case vallves 59 and 64 are closed and the saline solution, chilled in coils 51 of chamber 6, is circulated by pump 58 through valved line 12 and as required to Chillers 5 and 6 through line Kill and valved line Gl respectively. The saline solution is returned to coils 61 for further chilling through valved lines 62, 63 and 1l.

The illustrated apparatus of Figure 1 has 4been above-.described as it is employed for the production of highly purified benzene from benzene-containing mixtures after adm-ixing therewith as a diluent liquefied butane, or the like, whose boiling point is substantially lower than that of the benzene in said admixture. The apparatus, with `some modifications, lends itself to the use of diluents boiling at temperatures substantially higher than Adoes benzene. An example of such idiluent is Stoddard solvent, a petroleum deriva- Ative well known to the cleaning and dyeing industry, -haVin-g a .boiling range higher and a freez- -ing temperature lower than that of benzene. Stoddard solvent has a boiling range substantially between 140 C. and 210 C. and is produced to meet definite .standards as set forth in Commercial Standard CSS-40 of the Bureau of Standards,

United States Department of Commerce. .Stoddard solvent, in like manner to lthat ypreviously described, may .he admixed with a benzene-.containing mixture, the .admixtur.e=1chi1ledto a flowable slurry by .any pref-erred method and the slurry ilowed through the apparatus continuously to produce highly puried benzene, excepting only that in the final separations, in the fractionati-ng columns, the :Stoddard solvent which in this case is the higher boiling constituent is drawn off at .the bottom.

The .benzene-containing mixture :and the Stoddard isolvent'are flowed from their respective storage tanks through the chillers, separator, -melter and to the fractionating columns -as `'shown -by .the arrows and as previously described. In fractionatingcolumn i2 the highly puried benzene leaves as a vapor through line 35, is liquefied in condenser '36 .and flows through .line 31 to receiver 38. Pump 4Q sends the benzene to a storage tank (not shown) through valved line 4| while the necessary reux is taken loi .through valved 'line y42. With this method of operation :valve 43 is closed. The

Stoddard solvent is withdrawn from the bottomA of column Il2 through line 45, cooler 46 and through Avalved pump line 4.1 to line 44 whence it is returned to Stoddard solvent storage tank I for further use in the process. In fractionating column l5 .the benzene impurities, containing some benzene, are removed as vapors through line 39, and are again liquefied in condenser l1 and owed to a suitable storage tank v(not shown) through valved line 5i), valved line 5l 'being closed. The Stoddard solvent is withdrawn from column i5 yand returned to storage tank l, through line 53, cooler 54 and valved line 56, valved li-ne 55 being closed.

Referring now to IFigure 2 there is shown apparatus whereby a slurry of benzene crystals suspended in mother liquor is formed :by the evaporation of a portion of the diluent from the admixture. .In the operation of 'the apparatus by this method .some low boiling diluent such as propane is highly effective. 'Proportioning .pump3 draws the v.benzene-containing mixture and the liquefied propane from their respective storage tanks and sends them in parallel lines to chiller 6, as shown by the arrows, excepting that the to-.be-treated benzene in this modification receives a preliminary chilling, insuilicient for crystallization, however, in heat .exchanger 4, the liquefied propane flowing directly to chiller 5, wherein `a portion of the propane is evaporated through valved line Sii thereby chilling the remaining liquefied propane flowing through said chiller. For improved operation, because of pressure conditions `in Chiller 5, a pump `81 is employed to transfer .the chilled .propane from chiller A5 to 7ch-iller .6 and separator 8.

The benzene-propane admixture flows., as

shown by the arrows, through `:Chiller 6, Where the evaporation of a further portion of the propane from the admixture through valved line 88 completes the chilling necessary for the formation of a lowable slurry. The so-ormed slurry then passes into separator 8, where the mother liquor is withdrawn and the 'benzene crystals .are washed, the latter lbeing discharged into lmelter 9. where a stream of water in the melter jacket by indirect Contact melts the crystals. T-he now liquid benzene is 'sent 'to fractionating column vI2 where residual propane is separated from' the highly purified benzene, the former leaving column 12 as a Vapor through valved line 35 for further purification in column I5.y The purified benzene withdrawn from the bottom of column I2 is sent to a benzene storage tank (not shown) after being withdrawn therefrom through cooler 89 by valved line 9U.

The propane that is evaporated for chilling purposes is withdrawn from chillers 5 and 6 through valved lines 86 and 88 respectively by compressor I8 and returned after liquefaction in condenser I9 to propane storage tank I for further use in the process, said tank I also receives the propane separated from the benzene impurities in fractionating column I5 after proceeding according to the method described for Figure 1. The necessary reflux for fractionating columns I2 and I5 is supplied by proportionine` pump 3 through valved lines 9| and 92 respectively. The benzene impurities, which may oontain som'e benzene, are withdrawn from column I5 through cooler 93 by valved line 94 and may be sent to a motor fuel storage tank `(not shown) in those instances where relatively pure benzene is being processed.

If a sufficient amount of propane is employed to chill the benzene-propane mixture to a temperature of minus degrees to minus 30 degrecs Fahrenheit, then from 70 to 80 percent of the purified benzene may be removed from the bottom of the fractionating still I2. The remaining to 20 percent of the benzene will be a ben,- zene residue which could be utilized in motor fuel. This impure benzene may be removed with the mother liquor from the bottom of fractionatinf.r still I5 through a valved line 95.

In case it is desired to remove from 50 to 60 percent of the benzene as purified benzene by crvstallization.- then the amount of propane which is to be used (about 0 6`lbfof propane per lb. of benzene) will be sufficient to provide a slurry for flowing the crystals in the mother liquor and the temperature of chilling will b e'in the neighborhood of zero degrees Fahrenheit. The purified benzene in such cases will be removed from the bottom of the fractionating still I2 and the mother liouor containing from 40 to percent benzene will be removed from the base of the still I5 through a line 96 by which the impure benzene is introduced into the mid-por tion of a fractionating still 91. As the impure benzene flows through the line 96 isopropyl alcohol is introduced in to the line through a valved line 98. The alcohol forms an azeotropimmixture with the benzene in the mother liquor and this benzene-alcohol azeotrope is distilled overhead and passes through a line 99 into a condenser IUIJ and then flows into a receiver |92. Impurities of the benzene or the mother liquor which contains from 10 to 15 percent of benzene is removed from the base of the still 9'! through a valved line |03 and may be used as a motor fuel product. The benzene-isopropyl alcohol azeotrope flows from the bottom of the receiver I 02 through a line |04 to a separator |05. As the azeotropic mixture flows through the line water is added through a valved line |06 in sufficient quantity to dilute the alcohol to such a degree as to separate the alcohol from the benzene. The dilute alcohol will be withdrawn from the separator through a valved line IIJ'I and the purified benzene is withdrawn from the separator through the valved line |08.

The following examples are illustrative of the results obtainable by operation of the abovedescribed methods:

Example 1 A benzene product distilling, in standard Barrett apparatus, between the temperatures 4'79.8 C. and 84.4 C. and having a solidification point of substantially 4.6" C. was mixed with commercial-grade liquid butane in' the proportions respectively of about 3 to 1. The admixture was thereafter passed to a chilling apparatus where it was cooled to about minus 12 degrees C. to form a slurry, said slurry being passed to a. centrifuge wherein the benzene crystals were separated from mother liquor. The so-separated crystalline product was washed with a 'small amount of chilled liquid in the centrifuge, and

subsequently melted and a 60 percent yield of Eample 2 A benzene product distilling, in standard Barrett apparatus, between the temperatures 79.8 C. and 84.4 C. and having a solidication point of substantially 4.6 C. was mixed with Stoddard solvent (Commercial Standard CS3-40, Bureau of Standards, U. S. Department of Commerce) in the proportions respectively of about 4 to 1. The admixture was thereafter passed to a chilling apparatus where it was cooled to about minus 6 degrees C. to form a slurry, said slurry being passed to a centrifuge wherein the benzene crystals were separated from mother liquor. The `soseparated crystalline product Was washed with a small amount of chilled Stoddard solvent in the centrifuge, and subsequently melted and distilled to remove residual Stoddard solvent therein and a 43 percent yield of a benzene product having a solidification -point of 5.55 C. was obtained.

Example 3 A benzene product distilling, in standard Barrett apparatus, between the temperatures of 79.8 C. and 84.4 C. and having a solidification point of substantially 4.6 C. was mixed vwith commercial-grade liquid propane in the proportions respectively of about 4 to 6. The admixture was thereafter passed to a chilling apparatus wherein was maintained a pressure of about 2.7 lbs. per square inch gauge, thus allowing about 83 percent of the liquid propane present in the admixture to vaporize and thereby cooling the admixture to about minus 4 C. to form a slurry, said slurry being passed to a centrifuge wherein the benzene crystals were separated from mother liquor. The so-separated crystalline product was Washed with a lsmall amount of chilled liquid propane in the centrifuge, and subsequently melted and a 65 percent yield of a benzene product having a solidiiication point of 5.50 C. was obtained.

Example 4 Y A benzene product containing about percent benzene and about 2.5 percent each by volume of cyclohexane and 2,2,3-trimethylbutane and having a solidification point of substantially 3.3" C. was mixed with isopropyl alcohol in the proportions respectively of about 4 to 1. The admixture was thereafter passed to a chilling apparatus where it was cooled to about minus 5 C. to form a slurry, said slurry being passed to a centrifuge wherein the benzene crystals were separated from mother liquor. The so-separated crystalline product was washed with a small amount of chilled isopropyl alcohol in the centrifuge, and subsequently melted and extracted 111 with Water to remove residual isopropyl alcohol therein and a 54 percent yield of a benzene vpreduct having av solidiiica-tion point of 557 C. was obtained.

An advantage of the. present improved process resides inpartl inthe fact that the miscible liquid added to the impure benzene product at the process-inlet functionsy as a solvent-diluen-t. and a sort of extraction agent that' facilitates ushing the impurities of the benzene from thef surfaces of the crystallized product. Thev misoible agent also acts as a carrier for benzene crystals, resulting. in higher recoveries of benzene and ay purer product than would be possible with substan-l tially the same degree of crystallization of the benzene itself and furthermore makes it possible to perform the Various process steps in a con tinuous manner.

The present process is suitable for employment in the separation of a widerange of relatively easily crystallizable hydrocarbons and a specific procedure will vary accordingly as the composition of the initial. product varies. The yields, as are obvious from the hereinabove stated examples, naturally will also vary with the required degree ofv purity of the product the procedure isy employed to produce. The disposition. of the impurities removed from the rene-d product may be various and is Within the skill. of the art, being determined largely by the nature of the residual liquor.

This application is a continuation-impart of my application Serial No. 373,291 filed January 6, 1941, now abandoned for Method of refining light oil products. which is co-pending; herewith..

The invention as hereinabove set forth is embodied in particular form and manner but may be variously embodied Within the scope of the claims hereinafter made I claim:

1.. A method of recovering benzene of. a high degree of purity from impureV benzene comprising mixing impure benzene with a solvent in which the benzene and. the impurities in the benzene are readily dissolved and Which does not form an azeotrope with ben-zene, chilling, the mixture to crystal-liza a major portion of the benzene While thoroughly stirring the mixture to produce ne grained crystals, separating the crystallized benzene from the mother liquor,

r2* melting and distilling' tlf'ief benzene to.` recover a puried benzene, addi-rtg: a benzene azeotrope sel-ventto the moth-er liquor, distililing thev mixture toV separate, a. benzene azeotrope from imvpurities and treating the benzene azeotrope to separate a purifed benzenetherefrom;

2'.. The method defined: in claim 1 in which a low boiling parairr. hydrocarbon is; usedv asy the crystaliimg solvent.

3i, 'Ifile method defined2 in4 claim 1 which the azeotmpa solvent is'l an. aliphatic alcohol.

, The method defned in claim 1 in which isopropyl alcohol is used as the azeotrope solvent.

5. The. method dened' incl-aim I in which from. 5a: to 60 percent by volume of the benzene isz separated bycrystallization and from 30-to140 percent by volume of benzene is separate-dl by azeotropic distillation.

6. Thev method defined: in claim 1 in. which the volume of azeotrope solvent added to the mother liquor isabout, ene-third of the volume oi benzene in the mother liquor..

7. The method defined in claim 1 in which 15 te 2Q percent. by 'volume of propaneto the Volume of impure benzene is used as the crystallizsationV solvent and aboutl 33%. percent. by volunie of isopropyl alcohol to the Volume. of benzene in the mother liquor is, usedy for the azeo.- trope distillation separation- 8.. The method dei-"ined in claim. 1 in. which ,the solvent is separated. by distillation from the benzene and mother liquor and returned for reuse in treating, more impure benzene.

WILLIAM O. KEELING.

REFERENCES CITED- The: following references are of record in the me oi this patent:

UNI'IED STATES PATENTS 

